In recent years there have been many studies on the use of electromagnetic waves for energy transmission between two separated locations. It has been found in these studies that there is a need for a cost-effective means for reception and conversion of electromagnetic power to direct current power which is suitable for environments in which the radiated waves from the reception/conversion system could degrade the performance of other electronic systems within the range of the rectennas. An exemplary application of the reception/conversion system in which this need arises is the provisioning of 30 KW or more of propulsive and communications payload power for lightweight electrically-powered aircraft. In operation, such aircraft would circle over fixed ground antenna systems, transmitting power in the 2.4 to 2.5 GHz microwave Industrial, Scientific and Medical (ISM) band, for continuous periods of weeks or months at a time, relay communications signals between separated locations, or carry out other missions, based on long endurance aircraft platforms. It is, of course, of paramount importance to ensure that radiated emissions from the rectenna subsystem are not such as to degrade the performance of payload and aircraft electronics, or other ground or airborne electronics.
An applicant's copending application, Canadian Serial No. 557,680 filed on 16 Sep. 1988, which corresponds to U.S. patent application Ser. No. 07/124,159, filed Nov. 23, 1987 and now U.S. Pat. No. 4,943,811, describes a dual polarization power reception and conversion system. This system uses a rectenna array configuration which consists of two orthogonally polarized arrays of thin film rectennas with novel format and particular element spacing. This system has been demonstrated to be highly efficient for reception and conversion of the power in a microwave signal wavefield, received over a wide range of incident angles. However, it has a significant drawback which may limit its use in various operating environments, namely its potential for generating electromagnetic interference. Because of the non-linear nature of the power conversion process (from the powering frequency .function..sub.p to dc) at each rectenna element, a natural by-product of this conversion is the generation of harmonics n.function..sub.p of the powering frequency (where n is an integer). In prior art rectennas, as described in the above-mentioned pending application and in U.S. Pat. No. 3,434,678, the only measure taken to reduce the levels of radiation of this harmonic power is the inclusion of a low-pass filter as a circuit element between each antenna terminal and non-linear conversion device, such as rectifiers, etc. When only this means for harmonic attenuation is employed, significant levels of harmonic radiation, especially at the 2nd, 3rd and 4th harmonics, are produced, as reported in an article entitled "Environmental Assessment for the Satellite Power System Concept Development and Evaluation Program--Electromagnetic Systems Compatibility" by Davis et al, U.S. Dept. of Energy, DOE/ER-0096, January 1981.
In addition, and of major significance with prior art rectennas, is the radiation of `intermodulation product` signals. In many locations in which rectenna arrays could be used, radio signals from other users of the spectrum, of a wide variety of frequencies .function..sub.c1, . . ..function..sub.cm and strengths, will be incident on the rectenna array along with the powering signal at a frequency .function..sub.p. Non-linear mixing of all received signals at the rectifying diode will result in the generation of spurious intermodulation signals at combination frequencies ##EQU1## Due to the high level of the powering signal, these intermodulation product signals will be radiated at sufficient amplitudes to interfere with the performance of other electronic systems. The only measure taken to reduce this radiation in the prior art rectennas is the previously-mentioned low-pass circuit filtering which, because of the severity of the problem, in many circumstances may be insufficient to reduce the level of intermodulation product signals sufficiently to comply with regulatory standards set for interference levels.